Method and apparatus for electrostatic separation



Feb. 11, 1969 H. L. BULLOCK 3,426,895

METHOD AND APPARATUS FOR ELECTROSTATIC SEPARATION v Filed Nov. 21, 1966 INVENTOR. zegy Z. 5(JLLOCK ATTOR NE YS United States Patent METHOD AND APPARATUS FOR ELECTRO- STATIC SEPARATION Harry L. Bullock, Oak Park Village, Ill., assignor to National Engineering Company, Chicago, Ill., Filed Nov. 21, 1966, Ser. No. 595,755 US. Cl. 209-129 9 Claims Int. Cl. B030 7/10 ABSTRACT OF THE DISCLOSURE This invention relates to a method and apparatus for electrostatic separation, and more particularly to the electrostatic separation of substances from a mixture of such substances in the form of free flowing particles having dissimilar contact potentials. As is usual, the purpose of such electrostatic separation is to recover an economically more valuable constituent of a mixture from other constituents thereof, or to effect a 'beneficiation as to the desired constituent of the mixture. In some cases, the purpose may be merely to separate a mixture into two, or more fractions, each of enhanced purity as to some particular constitutent.

My present invention constitutes some respects over the electrostatic separation method and apparatus disclosed in my Patent No. 3,143,492. Instead of employing moving conveying surfaces, such as feed rolls, conveyor belts and the like, my present electrostatic separating equipment employs stationary particle-receiving and particle-directing surfaces so arranged and constructed that the flow of material through the equipment is effected solely by the action of gravity and the volume of flow is automatically coordinated with the volume rate of feed of material to the equipment.

By the use of stationary, smooth particle-contacting surfaces rather than moving conveying surfaces, there is less abrasive Wear, which may be excessive in the case of hard ores and other materials having sharp edges and points. A further possible disadvantage of moving conveying surfaces is that such surfaces generate contact charges with all of the feed particles with which they come into contact. Such generated contact charges may interfere with eflicient separation. Additionally, some economy is made possible by the use of stationary particle contacting surfaces instead of power-driven rollers, conveyor belts and the like.

In place of power driven feed rolls, my present equipment employs particle accumulators, or collectors, sometimes referred to as contact feeders, for the substantially automatic feed of the particulate material from one to another of a series of stages at which electrostatic separation takes place. Such contact feeders take the form of an open-face collector into which the particulate material undergoing separation flows by gravity and builds up, if not already built up, to an accumulation or pile of particles presenting a particle-formed surface that slopes downwardly at an angle related to the natural angle of repose of the particles. Once such a sloping particleformed surface has been built up, slippage of particles along such surface automatically provides a feed of particles to the next successive electrostatic separation stage.

An additional advantage of such a contact feeder is an improvement in that contact takes place in a particle-to-particle fashion, rather than between the particles and the surfaces of the equipment. Thus, a maximum generation of charges on the particles is effected by particle contact with particle, and a minimum generation of contact charges on the particles is effected by particle contact with particle-receiving and particle-supporting surfaces of the equipment. The result is a more uniform particle charge because of freedom of transfer of charges from one particle to another by contact therebetween.

It is, therefore, an important object of this invention to provide an electrostatic separation system that is relatively more economical to install and operate than many heretofore known electrostatic separation systems, and that embodies novel and improved contact feeder means that function substantially automatically to ensure the smooth and relatively uniform feed of the particles undergoing electrostatic separation in the passage of such particles from one stage to another of the system.

Other and further important objects of this invention will become apparent from the following description and appended claims, and by reference to the drawings, in which:

FIG. 1 is a vertical end view, more or less diagrammatic and partly in section, of a multi-stage electrostatic separating apparatus provided with contact feeders and embodying my invention;

FIG. 2 is an enlarged view in vertical section of the uppermost contact feeder illustrated in FIG. 1;

FIG. 3 is a view in vertical section of a modified form of contact feeder; and

FIG. 4 is a still further modified form of contact feeder in vertical section.

With reference to FIG. 1, the electrostatic separation apparatus here illustrated is designated as a Whole by the reference numeral 10 and comprises a plurality of electrostatic separation stages indicated at 11, 12, 13 and 14. Said apparatus 10 includes at the top thereof a downwardly sloping smooth-surfaced guide member 15 onto which is fed a particulate mixture for downward flow therealong of the particles in a stream 16. The source of the stream of particles 16 is not shown but may take the form of any suitable feeder, such as a vibrating feeder, pair of rollers, 'or the like, as illustrated in my patent No. 3,143,492, mentioned previously herein. Whatever feed mechanism is used in connection with the apparatus 10, it should preferably be such as to impart contact charges to the particles of the feed material or the feed material can be charged while flowing as a steam to or over the guide member 15. Since the source material will be a mixture of substances of dissimilar contact potentials, the particles making up such source material will assume individual charges in accordance with their respective contact potentials.

A contact feeder indicated generally by the reference numeral 17 is positioned adjacent to, but spaced from the lower end 18 of the guide member 15 so :as to receive the stream 16 of particles flowing downwardly over the surface of the guide member 15. Said contact feeder 17 includes a back wall 19 and a lower horizontal wall 20 extending inwardly from the lower end of the back wall to form therewith an open-face collector for the accumulation and buildup of a pile 21 of the particulate feed material supplied thereto -by the stream flow 16. While the contact feeder 17 is here shown diagrammatically, it will be understood that its length dimension will be related to the width of the guide plate 15 and that end Walls will be provided in any physical embodiment of the contact feeders of my invention to provide an open-face accumulator, or trough, capable of receiving the full width of the stream 16 without endwise spillage of particles from the pile 21.

As the pile 21 of particles builds up during operation of the aparatus, the particle-formed exposed surface 22 of the pile assumes an angle of slope that is related to the natural angle of repose of the particles making up the pile 21. As the angle of slope of the particle-formed surface 22 tends to exceed the angle of actual repose of the particles, slippage of said particles along said surface will take place and there will be a flow, as indicated at 23, downwardly over and beyond the forward terminating edge 24 of the horizontal wall 20. As the operation of the apparatus continues, a state of substantially uniform volume of flow will be reached that is equal to the volume of flow represented by the stream 16 that feeds the pile 21. Thus, if the volume rate of feed of the source material delivered to the guiding member is properly regulated and controlled, the feed from the pile 21 forming the downward flowing stream automatically regulates itself to the same volume rate of flow.

Each of the electrostatic separation stages 11, 12 and 13 is identical, so that only one stage need be specifically described, the stage at the top of the stack and identified as stage 11 being chosen for such specific description.

Such stage 11 comprises a divider 25 formed of a pair of diverging planar members 26 and 27 joined along their upper edges to form an apex 28 from which the members 26 and 27 diverge downwardly and outwardly at such an angle of slope as to slow the rate of fall of the particles and to avoid any bounding of particles. The member 26 forms the relatively shorter leg and the member 27 forms the relatively longer leg of the inverted V-shaped divider 25 for a function which will be made apparent as the description proceeds. Said apex 28 is so arranged as to be spaced below and somewhat inwardly (toward the right as viewed in FIG. 1) of the terminating edge 24 of the horizontal wall 20 of the contact feeder 17. The stream of particles 23 leaving said edge 24 will thus flow downwardly toward the apex 28, but since the terminating edge 24 is directly above the shorter leg 26, the stream 23, if only under the influence of gravity, would fall against the upper portion of the shorter leg just below the apex 28 and none of the stream would fall against the longer leg 27. In normal operation, however, the path of the stream 23 is modified, as will now be explained.

An electrode 29 is positioned with its axis horizontally aligned and substantially at the level of the horizontal wall portion 20 of the contact feeder 17, and in spaced relation but operatively adjacent the path of the flowing stream 23 as it traverses the vertical gap between the end 24 of the horizontal wall 20 and the apex 28. An appropriate voltage is supplied to the electrode 29 through electrical connectors 30, such that said electrode exerts an attracting force toward those particles in the flowing stream 23 that carry charges responsive to said attracting force and that are to be separated out. As a result thereof, the attracted particles are deflected toward the electrode 29 and caused to fall onto and along the longer leg 27 of the divider 25, as represented by the particle stream 31.

The particles in the flowing stream 23 that are not attracted by the electrode 29, or that are repelled thereby, are separated out of the stream 23 at the apex 28 to fall against and flow as a stream 36 along the shorter leg 26 of the divider 25.

A second contact feeder, indicated generally by the reference numeral and generally similar in construction and arrangement to the contact feeder 17, is positioned as a part of the separation stage 12 to receive the flow 36 of the non-attracted particles from the lower end of the divider leg 26. The contact feeder 35 thus builds up and maintains a pile 37 of accumulated particles, with resultant formation of a particle-formed surface 38 having a similar relationship to the natural angle of repose as was explained in connection with the particle-formed sloping surface 22.

From the particle-formed surface 38, there is slippage of particles downwardly along said surface and a resulting flow of particles 39 toward the apex of the divider 40 in stage 12. A similar separation takes place there under the action of an electrode 41, with a resultant buildup of a pile 42 of particles in the contact feeder 43.

A similar repetition of the separation is effected in stage 13 by virtue of the similar arrangement of the divider 45, electrode 46 and contact feeder 47 making up separation stage 13.

In the lowermost separation stage 14, which is of somewhat different construction from the separation stages 11, 12 and 13, a divider 50 in conjunction with an electrode 51 serves to separate a falling stream 52 of particles from the contact feeder 47 into streams 53 and 54, respectively, made up of the attracted and nonattracted particles. The non-attracted particles in the stream 54 flow along the leg 55 of the divider 50 into a collecting trough 56, from which a screw-type conveyor 57 conveys the non-attracted particles to a point of collection (not shown).

With regard to the path of travel of the particles on the other side of the vertical stack, viz., those particles that are attracted by the electrodes 29, 41, 46 and 51, such attracted particles flow in streams over the longer legs 27 of the uppermost divider 25, 60 of the next-below divider 40 and 61 of the next lower divider 62. The longer legs of said respective dividers, viz., the legs 27, 60 and 61, extend downwardly in spaced relation above and laterally beyond the respective electrodes 41, 46 and 51, with the extremities 63, 64 and 65 of said respective longer legs arranged in vertical alignment. Accordingly, the particles flowing over the longer legs 27, 60 and 61, in a manner similar to that already described, fall freely by gravity beyond the extremities 63, 64 and 65 and unite in a common stream 66 that is collected in a trough 67 at the lower end of the bottom divider 50 on the other side from the collector 56. The stream of particles 53 flowing over the side of the divider 50 that is nearest to the electrode 51, viz., the leg 68, also flows into the collector 67 to be mingled with the flow of particles 66. A conveyor 69, similar to the screw conveyor 57, serves to convey the attracted particles from the collector 67 to a point of storage or of further handling (not shown).

As best shown in FIG. 2, which is an enlargement of the contact feeder 17 shown in FIG. 1, the guide plate 15 is preferably provided at or near its lower end 18 with a transversely extending cleat, or ridge 70, which provides an offset from the contact surface 71 of said guide plate 15. The purpose of said ridge 70 is to retard the flow of particles in the stream 16 that comes into direct contact with said surface 71 to maintain thereon a particle layer that shields the surface 71 from the abrasive action of the most rapidly flowing particles in said stream 16, as well as increasing the contact of particle with particle. A smilar ridge is positioned adjacent the lower end of each of the shorter legs, like the shorter leg 26 of the divider 25, of the successively lower dividers 40 and 62.

Although all of the structural members of the electrostatic separation apparatus 10, as shown in FIG. 1, are preferably formed of dielectric, or electrically nonconductive, material, such structural members can be coated or faced with dieelectric material to provide surfaces for contact with the granular feed material, rather than being formed wholly of solid dielectric material. Also, as will be understood, the showing in FIG. 1, being largely diagrammatic, omits any supporting structure, enclosure or the like that would normally be used in an installation of electrostatic separation apparatus such as shown.

In FIG. 3, a contact feeder indicated generally by the reference numeral 75, is provided with a filler block 76 positioned therein against the inner converging surfaces of the horizontal wall 77 and the vertical wall 78. Said filler 76 has a forwardly facing sloping surface 79 against which a buildup of feed particles occurs to form a pile 80 resting against the sloping surface 79. The pile builds up as previously indicated, until the slope of the outer particle-formed surface 81 exceeds the angle of natural respose of the particles, whereupon the particles slip over said surface to form a falling stream 82, similar to the falling stream 23. Feed of particles to the contact feeder 75 is over a guide plate 151: in all respects similar to the guide plate 15.

In FIG. 4, a modified from of contact feeder, indicated generally by the reference numeral 85, is illustrated. Said contact feeder 85 is formed of a single sheet of dielectric material, or dielectric faced material, that presents a generally concave inner surface 86 for receiving a pile 87 of particles. The surface 86 includes a generally vertical upwardly extending portion 88 and a generally horizontal forwardly extending portion 89. The latter is smoothly curved at its extremity, as at 90, to provide a smooth surfaced edge portion over which particles fiow in a stream 91 from the surface 92 of the pile 87. As before, the pile 87 is built up until the particle-formed outer surface 92 slopes at an angle equal to or greater than the natural angle of repose of the particles.

By the use of dielectric, or non-conductive, flow surfaces, contact changes on the particles are conserved, but since the charges are small, no appreciable thickness of insulation is required. Non-conductive plastic, ceramic and other insulating coatings of no appreciable thickness can be used, with emphasis placed on their abrasion resistance.

The method of operation of the electrostatic apparatus above described will be clear from the description already given of what happens at the various stages of the flow of the feed particles through the apparatus. In general, the method is most satisfactorily carried out in connection with particulate material that is in free-flowing granular, or particle form. Satisfactory results have been obtained when the feed is of a particle size such that it is not substantially finer than 200 mesh nor coarser than 6 mesh, or more preferably, has been prescreened' into fractions of 12+4() and 40+200 mesh, expressed in terms of screen mesh numbers for Tyler standard screen scale sieves.

The method intended to be practiced by the use of the apparatus already described herein is best suited for the separation of substances that have effectively dissimilar contact potentials and that, prior to being fed into the apparatus of my invention, have been appropriately charged by contact techniques well understood in this art. Such charging can be carried out in the manner and with the use of equipment such as the vibratory feeder, illustrated and described in my aforementioned patent No. 3,143,492.

Upon the feeding of such precharged feed material into my equipment, the flow of the material through the several stages is automatically balanced with the rate of feed of particles to the first stage, even though the rate of flow volume of the feed is varied over a wide range. Such a balance of flow volume rate through the electrostatic separator with the flow volume rate of the feed to the separator results from the use of the contact feeders of my invention. Within the limits of feed required for the operation of my electrostatic separator on streams of particles of a stream thickness of about /a inch or less, such flow through the separator will automatically equal the feed to the first pile of particles 21 and will continue to do so as long as the feed is maintained. By metering the feed, the flow of particles in the successive separation stages can be held at a flow volume rate best suited to electrostatic deflection of the electrode-attracted particles from the falling stream at each stage.

The apparatus of my invention is so arranged and constructed that a large proportion of the contact charges on the particles in the flowing stream of particles through the apparatus is generated, maintained or adjusted automatically by contact of particles with particles. There are no charged moving surfaces to which the particles would be attracted and have to be scraped or wiped off, and no power is required for moving the feed through the apparatus since that is accomplished solely by the action of gravity.

It will be understood that modifications and variations may be effected without departing from the scope of the novel concepts of the present invention.

I claim:

1. In a method of electrostatically separating substances of dissimilar contact potentials from a charged particulate mixture of such substances, the steps which comprise:

feeding said charged particulate mixture into a gen erally downward predetermined flow path that includcs a series of vertically spaced pairs of diverging flow paths, each pair being in the form of an inverted V having an apex to divide the flow of particles into separate stream portions,

at one corresponding side of each said diverging paths,

directing corresponding separate stream portions against stationarily supported accumulations of particles having exposed particle-formed surfaces sloping toward the corresponding side of the apex of the next below pair of diverging paths at an angle of at least equal to the natural angle of repose of particles thereon, thereby causing particles to flow therefrom solely by the action of gravity in unsupported falling streams toward said one side of such apex and exerting an attracting force of sufiicient differential potential from the other side of said diverging paths toward each of said falling streams to cause thereby attracted charged particles to separate from said streams and fiow downwardly along said other side of said diverging paths while the non-attracted particles flow as separate stream portions downwardly along the corresponding one side of said diverging surfaces.

2. The method as defined by claim 1, wherein,

all of the attracted charged particles flowing downwardly along said other side of said diverging paths are collected separately from the non-attracted partic es.

3. The method as defined by claim 2, wherein,

each of the particle-formed surfaces is built up and maintained by the flow of particles toward a particlesupporting surface adjacent to the one side of said diverging paths extending into the vertical space between successive pairs of said diverging paths and terminating vertically above an apex of the nextbelow pair of diverging paths.

4. The method as defined by claim 2, wherein the particulate mixture is composed of free-flowing particles and said particle-formed surfaces are formed by a piling up of particles until a natural angle of repose is reached and maintained by a continued slippage of particles along the resulting sloping surface of particles,

whereby upon continued operation of the method the flow output of particles from a sloping surface auto- ;natically equals the flow intake to such sloping surace.

5. The method as defined by claim 1, wherein,

the continued flow of the particulate mixture fed into the predetermined flow path is maintained solely by the action of gravity until the lower end of the flow path is reached.

6. Apparatus for effecting electrostatic separation of w substances from a mixture of such substances in the form a of free flowing particles having dissimiliar contact potentials, comprising:

a stack of inverted V-shaped dividers arranged in spaced vertical alignment and having shorter legs facing toward one vertical side of said stack and longer legs facing toward the other vertical side of said stack,

contact feeder means providing stationary particle supporting surfaces along said one vertical side of said 7 stack with generally horizontal portions of said supporting surfaces extending into spaces between successive dividers and having edges terminating above the apex and adjacent the shorter leg of the nextbelow divider for building up and maintaining a pile of said particles until a forward face of said pile is formed that slopes downwardly at substantially the natural angle of repose of the particles forming the face, and

electrodes positioned along the other vertical side of said stack each in operative spaced relation to one of said edges and nearer to the longer leg of the nearest one of said dividers to attract and deflect particles falling over said edges toward said electrodes andthereby cause the attracted particles to flow along the surfaces of the longer legs of said dividers.

7. Apparatus as defined by claim 6, wherein said surfaces include a back wall forming with each of said extending portions an open front collector.

8. Apparatus as defined by claim 7, wherein, each of the dividers in said stack has its relatively short leg sloping toward the next-below of said particle supporting surfaces to deliver particles thereonto and has a relatively longer leg sloping laterally beyond and in spaced underlying relation to an electrode for flow along said longer leg of particles attracted by such electrode,

the construction and arrangement of such dividers and particle-supporting surfaces being such that flow of particles through said stack is maintained solely by gravity.

9. In an electrostatic separation apparatus, a contact feeder comprising a generally vertical back wall portion and a generally horizontal lower wall portion extending from said back wall portion and having a free front edge,

said walls together forming an open-faced receiver and collector of particulate material to build up and maintain a pile of such material until a forward face of said pile is formed that slopes downwardly at substantially the natural angle of repose of the particles forming said face,

means for directing particulate material toward said feeder to form and maintain said pile and to cause a slippage flow of particulate material along said forward face and the discharge thereof over said free front edge,

electrostatic separator means including an electrode spaced horizontally from said free front edge, and

particle receiving and separating means operatively associated with said electrode to receive said discharge of particles over said free front edge.

References Cited UNITED STATES PATENTS U.S. Cl. X.R. 

